A sed syntax

On Wed, 23 Jun 2010 05:22:35 -0500
Mike McCarty <Mike.McCarty at sbcglobal.net> wrote:
So much detailed answer, I can post it into my blog as a detailed
manual. too many thanks.
> littlebat wrote:
> > Hi,
> > I am learning LFS BOOK:
> > http://www.linuxfromscratch.org/lfs/view/6.6/chapter05/adjusting.html> >
> > Below is a sed syntax I can't understand and haven't found a place
> > to learn it.
> > <code>
> > sed -e "/^\*cpp:$/{n;s,$, -isystem /tools/include,}"
> > </code>
>> You already got a good answer, perhaps a little more detail helps...
> I'm no sed expert, but this is the way I read that command.
>> sed the command
>> -e means "execute this little program which follows"
>> " the quotes are necessary to keep the shell from
> trying to do stuff with what's here, and to make
> what follows "all one argument" to the program
>> / sed looks at the first character, and takes that
> to be the "delimeter". So, everything from here to
> the next "/" is the "address" sed will use to select
> lines from the file; the program gets executed on lines
> which match this pattern, all other lines pass through
> unchanged
>> ^ this indicates that the pattern must start at
> the beginning of the line
>> \* we have to "escape" the "*", or the shell will try to
> put file names in there, hence the "\" to make this
> a literal "*"
>> cpp: more string to look for
>> $ this says that when we've matched what went before,
> we must next find end of line, so, the entire line
> must be "*cpp:", so the command gets executed only
> on lines which contain "*cpp:" and nothing else
>> / here's the other delimeter "/" which ends the "address"
>> { this tells sed that what is contained is the script to
> execute, when we find a matching line; we do so up to
> the closing "}"
>> n Read/append the next line of input into the pattern space
> IOW, print what has been matched so far ("*cpp:") and
> then work on the next line
>> ; end of "n" command, so all we print is just "*cpp:"
> we use ";" to put multiple commands together, so this
> separates the "n" command from the "s" command
>> s now we start a "substitute" command
>> , this is taken by sed to be the delimter of the string
> to substitute for; this could be any character, like
> the "/" above; the "s" command wants
>> s<delim><string to find><delim><string to sub><delim>
>> where <delim> may be any character you like, but all three
> must be the same. In this case, ","
>> $ the pattern we are going to substitute for is end of line...
>> , ... and nothing else, the second "," matches the one above
> and ends the search string
>> -isystem /tools/include
> this is the string to substitute at end of line
>> , here's the third delimeter
>> } this marks end-of-command
>> " this is the matching quote for the shell to see
>> HTH
>> Mike
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littlebat